Mark IV Tank: Hinged Sponsons, Unditching Beams and a 1940 Car Crash

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In early 1917 Albert Gerald Stern, head of the tank supply department, halted battle tank production to develop a new engine and transmission, but with 200 tanks promised by April 1, 1917, he dropped the redesign and pushed a modified Mark I into production by May. The Mark IV had armor up to 12 millimeters to stop German armor-piercing bullets, a fuel tank moved to an armored box at the rear, and sponsons that swung inward on hinges so the tank fit on a railway car. Of 1,220 built, 420 were males with two six-pounders, 595 females with five machine guns and 205 unarmed tenders. The six-pounder sponsons were asymmetric to fit the gunner and loader, and only the right gun could fire straight ahead.

Eight men worked beside an unshielded engine at up to 120 degrees Fahrenheit amid carbon monoxide. Crews chose the Lewis gun's 96-round pan because Hotchkiss strips had to be cut to 14 rounds, and six-pounder barrels were shortened after digging into the mud. Field crews added fascine bundles to drop into trenches and unditching beams chained to the tracks under fire. The tanks helped at Messines in June 1917, sank at Passchendaele after a 24-day bombardment, and massed at Cambrai in November 1917 with nearly 460 machines. In April 1918 at Villers-Bretonneux, Mark IVs fought German A7Vs in the first tank battle, while Germany fielded about 40 captured Mark IVs.

• A steel tadpole tail meant to cross wider trenches bent in combat and was abandoned.
• The Germans crewed captured tanks with 12 men and swapped in captured Belgian cannon.
• A female tank given to Ashford in Kent had its engine removed and housed an electricity substation.
• The tank Deborah, knocked out at Cambrai and buried in a crater, was dug up at Flesquières in 1999.
• In 1940 the tank Excellent from HMS Excellent was revived for home defense and crashed into a civilian car.